1. Field of the Invention
The present invention relates to a method of forming a MOS transistor, and more particularly, to a method of forming a MOS transistor to prevent damage caused by an RCA cleaning process.
2. Description of the Prior Art
Metal oxide semiconductor (MOS) transistors are probably the most important components of semiconductor products. They are generally categorized as P-channel MOS (PMOS) transistors, N-channel MOS (NMOS) transistors or complementary MOS (CMOS) transistors. A MOS transistor comprises a gate, a drain and a source. In general, the gate is first formed on a silicon substrate of a semiconductor wafer. Then, an ion implantation process is performed, using the gate as a mask, to form a lightly doped drain (LDD), or a source and drain (S/D) on the silicon substrate around the gate. Then, a cleaning process is performed to remove impurities or contaminants on the semiconductor wafer so as to ensure the material properties of subsequent processes.
Hundreds of thousands of PMOS transistors and NMOS transistors can be formed on the semiconductor wafer at the same time. In order to define which of the PMOS transistors and the NMOS transistors are to undergo an ion implantation process, a photoresist layer must be formed prior to the ion implantation process to cover a predetermined area of the semiconductor wafer. Each time an ion implantation process is completed, the photoresist layer must be removed and an RCA cleaning process must be performed to rinse the surface of the semiconductor wafer.
However, the RCA cleaning process uses specific proportions of H.sub.2 O.sub.2, H.sub.2 O, NH.sub.4 OH, or HCl, which readily react with silicon or silicon oxide. Consequently, after the RCA cleaning process, a portion of the silicon substrate, or a silicon oxide layer on the semiconductor wafer, is etched. This disrupts the planarity of the semiconductor wafer. It also changes the thickness of the LDD on the silicon substrate, which effects the performance of the MOS transistor. Therefore, improving the method of forming the MOS transistor to prevent the damage caused by the RCA cleaning process becomes an important issue.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 are cross-sectional schematic diagrams of a method of forming a MOS transistor 20 according to the prior art. The MOS transistor 20 of the prior art is formed on a semiconductor wafer 10 that comprises a silicon substrate 12, a gate oxide layer 14 positioned on the silicon substrate 12, and at least one gate 16 positioned on a predetermined area of the silicon substrate 12. First, a photoresist layer (not shown) is formed on the semiconductor wafer 10 to define an active region. Then, an ion implantation process is performed, using both the photoresist layer and the gate 16 as a mask, to form a doped layer 18 on the silicon substrate 12 around the gate 16. The doped layer 18 is used as an LDD or an S/D of the MOS transistor 20. Then, after the photoresist layer is removed, an RCA cleaning process is performed to remove impurities or contaminants on the semiconductor wafer 10.
Since the cleaning solution used in the RCA cleaning process reacts with silicon, a portion of the silicon substrate 12 around the gate 16 is etched, forming an uneven surface after the RCA cleaning process, as shown in FIG. 2. This decreases the size of the doped layer 18 and increases the series resistance from the drain to the source, which leads to slower operating speeds of the MOS transistor 20.
In order to solve the above-mentioned problem, another prior art method of forming a MOS transistor uses a silicon oxide layer to protect the surface of the semiconductor wafer 10 so as to protect the silicon substrate 12 around the gate 16 from being etched. Please refer to FIG. 3 and FIG. 4. FIG. 3 and FIG. 4 are cross-sectional schematic diagrams of another method of forming a MOS transistor 24 according to the prior art. First, a silicon oxide layer 22 is formed on the semiconductor wafer 10 to cover the surface of the gate 16 and the gate oxide layer 14. Then, a photoresist layer (not shown) is formed on the semiconductor wafer 10 to define an active region. An ion implantation process is the performed, using the photoresist layer and the gate 16 as a mask, to form a doped layer 18 on the silicon substrate 12 around the gate 16. The doped layer 18 is used as an LDD or S/D of the MOS transistor 24. Next, after the photoresist layer is removed, an RCA cleaning process is performed to remove impurities and contaminants on the semiconductor wafer 10.
During the RCA cleaning process, a portion of the silicon oxide layer 22 is etched to form an uneven surface after the RCA cleaning process, as shown in FIG. 4. The silicon oxide layer 22 remaining on the semiconductor wafer 10 protects the surface of the silicon substrate 12 around the gate 16 from being etched. This prevents the decrease in size of the doped layer 18. However, during the ion implantation process, oxygen ions from the silicon oxide layer 22 diffuse into the doped layer 18, resulting in oxide enhanced diffusion (OED). This changes the doping concentration of the doped layer 18 and effects the performance of the MOS transistor 24. Furthermore, if boron ions are the primary dopants used in the ion implantation process, the boron ions in the doped layer 18 will diffuse into the silicon oxide layer 22 and thus decrease the concentration of boron ions in the doped layer 18.